R.f. transformer design for minimum coupling to shielding cover



Ot- 3 1961 H. T. PlcHAL l 3,003,129

ma TRANsFoRMER DESIGN FoR MINIMUM couPLmG To samwmc com Filed lay 25. 1960 INVENTOR.

. s ons 129 f mf. TnANsFoRMEii nsrGN FoR'MlNmUM coUrLlNG T SHIELDING com..

Henri T. Plchal, St. Petersblll'bFla., assigner to Generalv Dynamics Corporation, Rochester, NY., a corporation ofDelawnre Filed `May 23, 1960, Ser. No. 30,917 12 Claims.l (Cl. 336-87) The present invention relates generally to a newrand improved Yhighfrequency f transformer assembly and is more particularly concerned with a small, compact transformer assembly including an outer housing for magnetic and electrostatic shieldingbut constructed and to losses in order to provide high Q.

Bandpass transformers or coupled circuits are generally constmctedrgwith both windings wound on a single coil form usually cylindrical in shape so that the two windings are coaxial. Inorder to provideboth electrostatic and magnetic shielding `these windings are ,enclosed within ka metal vbox `or' can which may be either'cylindrical or rectangularin shape. If the shielding can is approximately the. same size as the` windings, substantial losses` arerinf troduced as a result ofthe coupling betweenthe shielding can and the windings. Therefore, when aihighV Qztrans-I former is'desired', the shielding can ispresently made -relativelyglarge krin order ltoreduce. this coupling.- At lowv radioffrequencies, ferrite coil forms or pottedassemblies to1increase Q by concentrating AJthe `mutual. ux linkagesvbetwcenthe windings vand reducing the leakage uxthmughthe shielding can. However, at higher radio` frequencieawfor-cxample', about 30 megacycles yand above: the advantages ofthe ferrite coil forms begin to diminish until'at about 40 to 50 megacycles higher Q values are obtained by usingrair core-inductances.` Thus, with a.

shielding'can'of givendimensions, the attainable yQ has a gxed 'ceiling' beyond which no improvement can befmadewithoutthe use of external circuit aids. Moreover,i the space limitations `of the equipmentin'which the trans-. former is to be usedv generally prevent `increase of :thersize of the `shielding can to improve the Q; Thus, shielded transformers are usually designed to vprovide a` cmnpr.

mise between the sizeof the `shielding `canand the Q.

It would, of course, be desirable to provideatransforrnerV but also free fromradiation to external circuits, thereby United Safe Of Patentedoct. 3,1961.

by the .slot is so formed that the magnetic loop axis parallel toboth windings, a result which is accom-` plishedby ,forming the slot longitudinally alongone sidek of thecanandextending at right anglesA to tliewinding axes-.ofthe coils.: The described construction retails.

slight' coupling`A 'betweenv theenclosed windings and` at rvthe' same time conserves ythe high Q values'of these Provision is alsofmade for adjusting the windings with respect to each other to varythe` coupling. The assembly is completed byfa ydetachable shielding clip which covers the-adjusting mechanism yand embraces thecan:v This clip maybe-removed to permit the couplingadjust-V mentbut, whenin position, it serves to eliminateradiation-from the kenclosed windings to the circuits external to ithecan,'-thus making rvthe' coupling, transformer of this invention `very well suited forruse in coupling raser-ies of amplifying stages.v Y

vThe invention, yboth, as toits organization andmanner of-foperation,.-togetherwwith further objects and advantages, `will best Ibeunderstood by reference to the following description :taken in conjunction with the: aceo|npanyingdrawingwherein: f

` FIG. `l yis ay perspeotivefviewV illustratingfan RF transformer' assembly. characterized bythefeatures of the present-invention;Y

FIG. 2 isI an lenlarged sectional view taken along a linesubstantially corresponding to the Aline Z-Z offFIG. 1;`.

` FIGt` 3 'saasectional view taken along ra corre.

Y spondng substantially totheline-B .in FIG; 2 'assum-` ing, of course-.that the latter shows the entire transformer4k assemblviand g f FIG. r.4 is afragmentarexploded, perspective view ilf` lustratingparticularly k-an :adjustable mounting `arrange.-

ment for one of the windings of the transformer assembly l Referring nowftothe-drawinga transformer'assem'bly. characterized by the `features ofthe present invention is., thereidentilied generally/by the reference. numeral 10y andincludesa of windings 11 and .12,one of which1V serves as theprimarywinding of thetransformer and the otherrofgwhichserves as` the secondary winding. 'It will" Y be recognizedthat either onewof thewindingsll or 12.@

may. @beused astheprimary winding.; Each vol? the wind-2 ings l11 Tand.12 includes a coilformedof a number Aoft turns woundeupon an appropriate coil form of low-lossl dielectric-for insulating -rmaterial preferably cylindricalv in shape. The4 coilform for the winding 11 risidentiied by.r

making this transformer especially well suited for use in'V coupling between stages in an amplier chain.y

f The invention has for another object the, provisionof a highQ transformerhavinga` pair of windings mounted within a shielding can for adjustment with respect to each other in order to alter the coupling therebetween with theadjustng mechanism being covered or shieldedl to prevent radiation of signals to circuits external to the shielding can.

Thev foregoing and other objects are realized in 4at:-

the reerencenumeral p13 :while that for the winding 12 is. indicated by4 vthe numeral 14.A The coil .forms 13and14` are `supported in spaced `apart positionslupon a shielding` can' '15 with ytheir, winding axes extending `parallel to. each form 1'4 are respectively glued or otherwise secured to the side walls ,16 and 17v of thecan asindicated at 18 and V19." The-*coil form `ltmay include 4an annular. collarorllangeU 24.seatingragainst the. side wall 16 and formed .adjacent an outer end 20 whichextends through an opening inthe.

Vwall 16-to permitaccess from the exterior oliv the shield-` ing can so-that. a core 21 may be adjustably threaded into or outoftthecoil form. ,i The winding ,11 may also besecuredin fixed position by gluingits ends to the .sidesY cordance with the present invention, by providing a transf former wherein the primary and secondary windings are wound with their respective axes parallel to one another. The shielding can takes the form of a relatively at, rectangularlyshaped can which provides a long loop coupling to the transformer windings, thereby reducingthe losses duer` to leakage tlux. A slot is provided along one:`

of the sides of the can for the purpose of further reducingthe coupling between windings causedby the presence ofthe can. To retain the benefits of magnetic shieldingn of theshieldingcan. However, in cases where it is delv sired to -providefor varying the coupling between the windings 11 and 12,the coil form 13 is mounted for ad? justmentvertically upony the shielding can 15. To end, thesidewall 16 is-providedwith an elongated open-4 ing 26 -for accommodating one end `28 of the coil form 13..' The end 28 ofthe coil form 13 includes an externally threadedportion yiitlnpiotr'ucling beyond the wall 16 to4 receive a nut 31 whichmaybe :tightened to lock the coil form 13 An annular collartSsontheycoilul form 13 seatsV against the side wall 16 so that when the nut 31 is tightened, the coil form 13 is clamped to the side wall 16 to provide a cantilever type support. The

nut 31, when loosened, permits vertical adjustment of the position of the winding 11 by moving the coil form within the opening 26. The winding 11 is lowered as viewed in FIG. 2 to increase the coupling between the primary and secondary windings and is raised to decrease the coupling. To prevent the coil form 13 from turning about its laxis the end portion 28, as is best shown in FIGS. 3 and 4, includes a flange 32 having diametrically opposed flat portions 33 and 34 thereon lying contiguous to the opposed side edges of the slot 26.

The shielding can 15, of course, is formed of conducting metal to provide the aforementioned electrostatic and magnetic shielding. In accordance with an important feature of the present invention, the shielding can be made very small without adversely affecting the Q of the trans former to any great degree. Specifically, the width of the can may be made approximately equal to the length of the coil forms 13 and 14 and the depth or thickness of the can may be made only slightly larger `than the diameter of the windings l1 and 12.` This desirable result may be attributed to the fact that since the height or longitudinal length of the shielding can is relatively long a relatively long loop coupling is provided to the transformer windings and, hence, the shielding can has little effect on the transformer Q. Thus, in one construction which has been found to provide satisfactory results windings having, a diameter of 0.25 inch were used in a shielding can having an inside depth of only 0.32 inch and the resultant reduction in Q; that is, the Q of the transformer without any shielding as comparedr with the transformer Q obtained with the shielding can; was found to be only about 5%. The lengths ofthe coil forms 13 and 14 are at least 1.5 times the length of the coil windings thereon in order to minimize coupling into the side walls 16 and 17 of the shielding can.

V In accordance with another very important feature of the invention, the side wall 16 of the shielding can 15 has a narrow slot 38 cut therethrough and extending from a point near the top of the can to a point near the bottom. This slot removes coupling between the windings 11 and 12 induced by the presence of the shielding can 15. In order to retain the benefits of magnetic shielding by the can 15, the shielding can is so shaped and the windings 11 and l12 are so oriented that the magnetic shielding loop has its axis extending parallel to the axes of all of the windings. .This means that the slot 38 should extend longitudinally of the shielding can and at right angles to the axes of the windings 11 and 12, i.e., along one of the side walls as illustrated in the drawing. The described construction yields a maximum magnetic shielding for a minimum amount of coupling between the shielding can and the windings for a lgiven volume. Thus, the windings may beassembled within a very small space while at'the samevtime retaining loose coupling therebetween and also,y

maintaining high Q values. The use of a slotV in the shielding` can in the manner described permits Athe coupling factor between the windings Vto be maintained at a low value even though the shielding can itself is relatively small. Y Y',

' The winding 11 may ibe of theltype employing an adjustable core indicated at 40 in FIG. 2. The core y40 may be adjustedby inserting an insulated screw driver or other adjusting tool through the end of the coil form 13 which extends through the opening 26 in the side wall 16 of the shielding can. l Without the presence Yof the holes 21 and 26 in the side wall 16, capacitors wouldbe formed be'-,

tween the adjustable cores of the windings and the shielding can. These capacitors would provide undesired coupling into Ythe side walls of the can. The holes in the end walls form 'al shorted turn for eddy currents caused by the magnetic flux induced in the shielding can and, hence, theyV cause an absorption of energy. `Since the opening elongated slot 38 it may be desirable in some constructions to eliminate the elongated slot. However, the elongated slot has the advantage of breaking the short circuit around the openings and, hence, reduces the eddy currents described above. The .useof the wide opening 26 opens the shielding effect of the can 15 and permits magnetic leakage therethrough. To avoid this magnetic leakage while at the same time retaining the aforementioned reduction in eddy currents, a metal clip or bridge 42 s placed over the opening 26. This clip includes a central web portion 43'having two integral arms 44 and 45, respectively, extending Aalnng the two side walls of the shielding can 15. The clip is dimensioned so that the arms 44 and 45 tightly embrace the can to prevent slipping and to provide low resistance electrical contact but the clip may be readily removed to permit adjustment of the core 40 or to alter the position of the winding 11 to adjust the coupling. If the clip tends to slip along the shielding can as may be caused, for example, by vibration, this clip may be soldered or spot welded very lightly without effectively bridging the slot or opening 26. An insulating layer 46 such as an insulating tape is secured to the inside of the central web 43 of the clip -to` prevent electrical contact and to avoid the introduction of excessive capacity between the clip and the core 40. Since the clip eifectively prevents radiation of the transformer signals, its use is particularly efective when the transformer 1,0 is employed to provide coupling between stages in a series of amplifiers. Y Y Y In view ofthe foregoing discussion, it will be recognized thatthe transformer assembly described may be made in avery small unit with the front and rear walls of the shieldingcan lying Very close to the coils or windings while, at the same time, retaining the electrical characteristics of a high Q transformer. Thus, the assembly is best suited Vfor use in installations where miniaturzation of components is the prime concern. It will also be recognized that the use of a high Q transformer permits a given selectivity or bandpass to be realized without using a large number of stages and without delicate adjustments. Thus, the use of the transformer of the present inventionresults in a reduction in cost of component j elements and at thesame time increases the stability and 26 breaks the Ymagnetic path in the same manner as the 75 reliability of the circuits in which it is employed.

While the present invention has been shown and described in conjunction with an illustrative embodiment thereof, many modifications will occur to those skilled in this art, and itis, therefore, contemplated by the `appended claims to cover any such modifications as 'fall within the true spirit and scope ofthe invention. p 1

What is claimed as new and desired to be secured by Letters Patent of the United States is:

' l. A coupling transformer comprising an elongated shielding can of substantially rectangular cross section, said shielding canhaving a width between two side walls considerably greater than the depth between the front and rear walls and having a height from top to bottom considerably greater `than the width, a primary windingV and a secondary winding extending parallel to each other within said can, each of said rwindings being wound upon tially the same diameter, eachk of said coil forms extending between said side walls and being substantially co-` extensive withV the, width of the can, each winding being wound about its coil form for adistance less than the length of the lform so that both ends of each winding are spaced from the side walls of the can, the depth of the shielding can being only slightly greater than thediameter of the coil forms, and an elongated slot extending lengthwise along one of the side walls of said can to minimize the coupling between the windings and the can, thereby providing a Vhigh Q coupling transformer of very small size. i

2. The apparatus deiinedby claim l wherein the elongatedslot receives an end portion of one ofsaid coil releasable means is released in order to permit adjustment of the coupling between the windings.

3. The apparatus dened by claim 1 wherein each of the coil forms carries an adjustable core and wherein said one side wall of the can is provided with at least` one opening to permit access to the adjustable coresbY an adjusting tool. v Y

4. Apparatus according to claim 3y wherein said one side wall is provided with a plairrof spaced apart openings, one for each of .said coil forms, yand said elongated slot extends along said one side wall and intersects both of said openings.

` y5. Apparatus according to claim l wherein a conducting clip is placed over said shielding can to cover at least a portion of said one side wall, said clip including arms respectively engaging the front and rear walls of thaencan and being electrically insulated `from said one side w rr6. Apparatus according to claim 2 wherein aconduct ing clip is placed over said shielding can to cover the Y portion of said one side wall adjacent said slot, said clip including arms respectively engaging the front and rear walls of the can and :being electrically insulated from said one side wall. f a

7. A coupling transformer comprisingan elongated shielding can of substantiallyrectangular cross section, said shielding can having a width between two side walls considerably greater than the depth between the front and rear walls and having a height from top to bottom 'considerably greater than ther width, a primary winding and a secondary winding extending parallel to each other within said can, each of said windings being wound upon a dielectric coil form supported within the can, each of said coil forms extending betweenr said side walls and being substantially coextensive with the width of the can, each winding being wound about its coil form for a distance less than the length of the form so thatrboth ends of. each winding are spaced from the side walls of the can, the depth of the shielding can being only slightly greater than the maximum transverse dimension of the coil forms, and an elongated slot extending lengthwise ralong one of the side wallsof saidjcan and perpendicular to the axes ofthe in order to minimize the coupling between the windings and the can, thereby providing a highy Q coupling transformer of very small size.

8. The apparatus defined by claim 7 wherein the elongated slo'treceives an end portion of one of said coil forms and wherein releasable means are provided for clamping said one coil form in position, said one coil form being movable within the elongated `slot when ther releasable means is released rin order to permit adjustmentof the coupling between the winding. 1

9. The apparatus defined by claim 7 wherein each of the coil forms carries an adjustable core and wherein said'one side wall of the canis provided with at least one opening to permit access to the adjustable cores by an adjusting tool. l0. Apparatus according `vto claim 9 wherein said one side wall is provided with `a pair of spaced apart openings,fone foreach of said coil forms, and said elongated slot extends along said one side wall and intersects both of said, openings. y f 1l. Apparatus according to claim 7r wherein a conducting clip is placed over said shielding can to cover al: ,i least a portion .of'said one side wall, said clip including arms respectively engaging the front and rear walls of the can and being electrically insulated from said one t References cned in the me of :his patent UNITED STATES PATENTS schone oet. a1. 1939 Verhoeven Aug. 26, 1958 

